When producing plate heat exchangers one generally uses thin foils of a suitable brazing material, which foils are placed between the heat exchanging plates, which are to be brazed together. The heat exchanging plates with the foils located between them form a plate package comprising the desired number of passages for the media, which shall exchange heat. The plate package is placed in a furnace and is heated to the temperature at which the brazing material melts. The brazing may take place under vacuum or in presence of an inert or active shielding gas, such as nitrogen, hydrogen, helium or argon or combinations thereof.
In order to obtain a joint by brazing it is required that the brazing material wets the surface of the objects which are to be brazed together and that the brazing material has a melting point that is below the melting point of the objects which are to be joined together.
If one uses a brazing material in the shape of a powder this may be mixed with a binder or may alternatively be added to the plates in two steps.
The brazing material may also be dispersed in a mixture of binder and liquid and be painted or sprayed on the surface of base material. Another way is to apply the binder firstly and the pulverized brazing material thereafter. In case a binder is used, the plate package is suitably heated in steps in such a way that the binder is vaporized, before the material reaches brazing temperature.
In order to secure a sufficient strength of the heat exchanger one strives to obtain perfect brazing joints, which do not contain brittle phases or cracks. Brittle phases and cracks constitute crack initiating sites for fatigue failure and may form conditions for formation of corrosion cells, which may cause serious faults in a heat exchanger. The resulting crack initiating sites may also give rise to a leaching of alloy elements to the heat exchanging media, which is unsuitable in brazed constructions for food applications.
When using active brazing material, that is brazing material which contains elements which lower the melting point, the risk that brittle phases are formed increases. This depends on processes, which affect the diffusion rate of the melting point lowering elements in and around the brazing joint. If the driving force for diffusion and the kinetic conditions are suitable the so called critical joint clearance increases, the joint clearance where no brittle phases develop in the joint.
The brazing materials, which are used today, have often good flowability and wetting properties in order to penetrate into crevices and achieve a good bonding to the base material. In plate heat exchangers, where the plates which are to be brazed together, have a press pattern with elevations and depressions it is usual that the brazing joints have the shape of a point. Generally one uses a brazing material in the shape of a foil of uniform thickness, which covers the whole plate apart from the port holes. This means that one uses a surplus of brazing material in order to have a sufficient amount of brazing material in the brazing joints. Since it is difficult to control the amount of brazing material in the brazing joint, the risk increases that the amount of brazing material will be too large in certain brazing joints. At which the risk for brittle phases increase.
The article “Alloys for brazing thin sections of stainless steel” by A. S. McDonald in Welding Journal March 1957 discusses which alloys may be considered suitable for brazing thin steel elements for example to heat exchangers. An ideal alloy should according to the author of the article be able to wet and flow over a stainless steel surface without any flux agent during brazing in a shielding atmosphere. It should not damage the base material by dissolving the same or penetrate into the material and the obtained brazing joint should have a good mechanical strength and be resistant to oxidation.
The article continues with the statement that the popular nickel based alloys which contain boron and which in other connections are very useful cannot be used depending on their dissolving and penetrating properties.